The Java Virtual Machineand Mobile Devices

John Buford, Ph.D.buford@alum.mit.edu

Oct 2003Presented to Gordon College CS 311

Objectives

• Review virtual machine concept• Introduce stack machine architecture and relate it to

CISC and RISC architectures• Present key features of Java VM• Describe use of JVM technology in mobile device

area

Virtual Machine

• A processor specification implemented in a software interpreter with the following characteristics:– intended to execute programs written in some high level

language– includes an instruction set, a register set, and a memory

model– usually designed for portability– frequently used inside compilers as an “intermediate

language” to which generic optimizations are made• Historical examples:

– Pascal P-Code, Smalltalk, Portable Scheme, etc.• Current examples:

– Java VM, Microsoft .NET Common Language Runtime

Virtual Machine

• A specification of a machine language and the CPU and memory organization

• Typically designed as a generic model• a “stack machine” is one such a generic model

• Inside view – a machine that can be simulated in software– application program opcodes stored in files produced by compiler– VM reads opcodes into an array– VM interprets each opcode one-by-one just as a hardware CPU

would– Each opcode is interpreted; the execution of the opcode causes

changes to other data structures in the program that represent other parts of the machine organization

Stack Machine

• A CPU architecture where all operations are performed on a stack

• Most instructions use implied addressing mode– so instruction size is short compared with other

architectures– But more instructions are needed for a complex

operation than in other designs

• Use of stack machines in some early computers– Burroughs B 5000 (circa 1960s)– HP3000 minicomputers (circa 1970s)– Stack machine has been more popular as a

virtual machine than as an actual hardware design

• Software implementation relatively easy– Most computers have a hardware stack

• Why not use an abstract RISC VM instead of a stack machine?

a[0]

4

object ref TOS

MAX

MIN

Implied Addressing Mode

• Register addressing mode (Intel)– ADD AX,BX

• Direct address mode– ADD Var1, Var2

• Implied mode– PUSH Var1 // destination is implied– PUSH Var2 // destination is implied– ADD // addends are implied

• Implied addressing gives shorter instructions, but generally more instructions for given operation

Stack vs CISC vs RISC

• You’re building a new distributed agent software platform that will run on every future computing device. You’ve decided to use a virtual machine.

• How would you compare the architectural choices:– CISC– RISC– Stack machine

Java Virtual Machine

• 32-bit stack machine• Floating point and integer data types• ~200 instructions• Most instructions are generic, but some are

related to the Java object model

Typical Java Runtime System

DynamicClass

Loaderand

Verifier

Classand

MethodArea

GarbageCollected

Heap

Support Code:

ExceptionsThreadsSecurity...

ExecutionEngine

NativeMethod

Area

NativeMethodLinker

Operating System

Native Methods.dll or .so

StandardBuilt-in

Java Classes

Applet orApplication Class Files

Execution Engine

Object Heap

Allocated

Free

Being Reclaimed

a[0]4

object ref TOS

MAX

MIN

System Thread

a[0]4

object ref TOS

MAX

MIN

Event Loop

a[0]4

object ref TOS

MAX

MIN

Application Thread 1

a[0]4

object ref TOS

MAX

MIN

Application Thread N

Classes

Constant Pool

Instruction Categories

• Arithmetic• Logical• Conversion operations• Control transfer• Function return

• Stack operations• Pushing constants on to the stack• Loading/storing local variables on to/from the stack

• Managing arrays• Method invocation• Manipulating object fields• Exception handling• Miscellaneous object operations• Monitors

Examples• Add Two values16 05 bipush 5 => 516 06 bipush 6 ..., 5 => 5, 696 iadd 5 , 6 => 11

• HelloWorldpublic class HelloWorld {

public static void main(String args[ ] ) {

System.out.println(“Hello world!”);

}

}

Inside “HelloWorld”javap -c HelloWorldCompiled from HelloWorld.javapublic class HelloWorld extends java.lang.Object

/* ACC_SUPER bit set */{

public static void main(java.lang.String []);public HelloWorld();

; call Object's constructorMethod void main(java.lang.String [])

0 getstatic #7 <Field java.lang.System.out Ljava/io/PrintStream;> ; #7 = index into the constant pool

3 ldc #1 <String "Hello World!"> ; push item which is a constant

5 invokevirtual #8 <Method java.io.PrintStream.println(Ljava/lang/String;)V> ; invoke the instance method

8 return

; Push the output stream and the string "Hello World" onto the stack,; then invoke the println method:Method HelloWorld()

0 aload_0 ; load the object ref onto the stack1 invokespecial #6 <Method java.lang.Object.<init>()V> ; invoke the object initializer method4 return

}

JVM Organization• 32-bit operand stack

– Longs and doubles take 2 entries• Stack frame, one for each executing

method– Local variables– Execution environment– Operand stack

• Local variables– Stored in 32-bit cells– 64-bit types use two cells

• Execution environment– Copies of registers prior to new call frame

(PC, frame ptr, TOS) – private data (implementation specific, e.g.

method and object ref, class table, etc)• Registers

– Program counter (PC)– Optop (TOS pointer)– Frame Ptr: reference to the execution

environment for the currently executing method

– Vars: reference to the local variables for the currently executing method

PC

Optop

FramePtr

vars

Stack

prev frame ptr

prev PC

prev TOS

local vars

prev method’s stack area

prev frame

MAX

MIN

JVM Organization (cont’d)

• Constant pool– Each class has a constant pool– Constants are stored in the class file produced by compiler– There are constants for each method and field referenced by an

object of this class

• Method area– Stores method byte code

• Example graphical simulator for VM at– http://www.artima.com/insidejvm/applets/index.html

Basic Structure of JVM Interpreter

read machine code from fileinitialize state of vmwhile(true) {

opcode = machine_code[pc];pc += 1;switch(opcode) {

case BIPUSH: break;case SIPUSH: break;/* etc. */

} }

Questions

• What issues are there in dealing with mixed datatypes in a fixed-width stack?

• How could stack corruption (accidental or otherwise) effect VM integrity and security?

• How could bytecode be written to adversely effect VM integrity and security?

• How does the VM prevent inadvertant stack corruption and bytecode errors?

• How does this impact performance?

Java on Embeddedand Mobile Devices

Cell Phones

PDAsSee http://www.mobiletechnews.com/

Telematics and Info Appliances

Features of Mobile Platforms

• PDAs– OS: Palm, PocketPC, Embedded Linux, Symbian– Processor: ARM, StrongARM, OMAP, Xscale, 68K– Memory: 16MB to 64MB– IO: WiFi, mini-keyboard/touch screen, display

• Cell Phones– OS: various real-time embedded OS and proprietary software,

Qualcomm BREW– Processor: ARM; Motorola, SHx– Memory: – IO: flash memory, keypad, audio, video

• Large variety of systems software and hardware– Java offers portability for applications to run – But J2SE is too resource intensive …

Java 2 Microedition

Java 2 Microedition Editionhttp://jcp.org

CLDC 1.0

MIDP 1.0

WMA 1.0

CLDC 1.1

MIDP 2.0

CLDC 1.1

MIDP 2.0

JTWI

CDC 1.0

FP 1.0

PBP 1.0

PP 1.0

CDC 1.1

FP 1.1

PBP 1.1

PP 1.1

CDC CLDC/MIDP

Optional JSRs

JSR 135 Mobile MediaJSR 179 Location APIJSR 205 WMA 2.0JSR 209 Adv Graphics and UIJSR 211 Content HandlerJSR 184 Mobile 3D GraphicsJSR 229 Payment API…

Further Information

• Java VM Specifications– http://java.sun.com/docs/books/vmspec/2nd-edition/html/VMSpecTOC.doc.html

• Java VM resources– http://www.artima.com/insidejvm/resources/index.html

• Open source JVM implementations– Kaffe (http://www.kaffe.org)– Wonka (http://www.acunia.com/wonka/001_about.php)

Summary

• Review virtual machine concept• Introduce stack machine architecture and relate it to

CISC and RISC architectures• Present key features of Java VM• Describe use of JVM technology in mobile device

area

Discussion